Radiotherapy is a therapeutic technique of destroying a lesion by irradiating the lesion with a therapeutic beam. Thus, unless a therapeutic beam is precisely radiated at the position of the target lesion, there is a possibility that normal tissues around the target lesion are also destroyed.
For this reason, first, CT (computed tomography) imaging is performed on a patient in advance of irradiation of a therapeutic beam in many cases, in order to acquire volume data inside the patient's body and three-dimensionally identify a position of a lesion.
Next, a treatment plan including an irradiation direction and irradiation intensity of a therapeutic beam is determined on the basis of the volume data inside the patient's body so that normal tissues are subjected to as little irradiation as possible.
Then, in an irradiation phase of a therapeutic beam, a bed on which a patient lying is moved and adjusted so that a therapeutic beam is precisely aimed at the lesion position identified with the patient volume data, the lesion position acquired through the treatment plan.
And then, immediately before irradiation of a therapeutic beam, confirmation is performed as to whether the aiming point of a therapeutic beam actually matches or not with the identified lesion position of the patient lying on the bed.
Confirmation of matching between the aiming point of a therapeutic beam and the lesion position is performed on the basis of matching processing between an X-ray projection image obtained by imaging the patient lying on a bed with the use of an X-ray imaging unit permanently mounted on a radiotherapy apparatus and a DRR (Digitally Reconstructed Radiograph) reconstructed as a projection image from the volume data used for the treatment plan. Specifically, confirmation of matching between the aiming point of a therapeutic beam and the lesion position is performed by determining whether the lesion positions of both match each other or not in the above-described matching processing.
Although many methods such as automation are proposed for the above confirmation, ultimately, an inspection engineer visually checks whether the aiming point of a therapeutic beam matches or not with the actual lesion position.
In some cases, irradiation of a therapeutic beam on a patient in radiotherapy is performed for several times to some dozen times over plural days changing a posture of the patient on a bed. Thus, confirmation of matching between an aiming point of a therapeutic beam and a lesion position is performed each time of irradiating the patient with a therapeutic beam, e.g., each day of performing irradiation.
Meanwhile, a position, an angle, and other imaging conditions of an X-ray imaging unit permanently mounted on a radiotherapy apparatus are changing with time. Thus, there is a problem that an X-ray projection image of a patient imaged by an X-ray imaging unit does not necessarily match a projection image reconstructed from volume data each time.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 2014-171763